The purposes of this project are to examine the contributions of central motor programming and afferent input in the control of arm movements in normal and parkinsonian subjects, and to study psychomotor performance of patients with central motor disorders. The first set of experiments records electromyographic activity and kinematics of limb position while subjects manually match a target display with either a skilled rapid or slow movement with a handle whose displacement controls a visual display or attempt to control the discharge of single motor units (MU) when the limb is passively moved. Movement, amplitude, presence of absence of visual feedback of position, and disturbances of the subject's movements are independent variables. Large movements and sustained rates of MU discharge are performed accurately independent of manipulation of the experimental variables but accurate performance of small movements and generation of single MU spikes becomes increasingly dependent on the absence of limb disturbances during movement. The second set of studies examined a variety of psychomotor variables from patients with Parkinson's disease. Tests of movement speed, reaction time, and gait abilities have provided measures that correlated with clinically determined fluctuations in drug efficacy.